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Difficulty: Very hardAlkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

Match each organic reagent setup or chemical process in Column A with its corresponding chemical reaction product or characteristic diagnostic observation in Column B.

  • Bubbling but−1−ynebut-1-yne gas into ammoniacal silver nitrate solution, [Ag(NH3)2]NO3[Ag(NH_3)_2]NO_3Formation of a white precipitate due to replacement of an acidic acetylenic hydrogen atom
  • Passing propenepropene gas into cold, dilute alkaline potassium tetraoxomanganate(VII) solution, KMnO4KMnO_4Decolourization of the purple solution accompanied by the deposition of a brown precipitate of MnO2MnO_2 and formation of a diol
  • Heating excess ethanol with concentrated tetraoxosulfate(VI) acid, H2SO4H_2SO_4, at 170∘C170^\circ\text{C}Intramolecular dehydration producing ethene gas as the major organic product
  • Controlled addition of cold water to calcium dicarbide, CaC2CaC_2Hydrolysis reaction producing ethyne gas and calcium hydroxide, Ca(OH)2Ca(OH)_2

Answer

Bubbling but−1−ynebut-1-yne into ammoniacal silver nitrate forms a white precipitate of silver but-1-ynide due to acidic acetylenic hydrogen replacement; passing propenepropene into cold dilute alkaline KMnO4KMnO_4 decolourizes purple MnO4−MnO_4^- forming a diol and brown MnO2MnO_2; heating ethanol with excess concentrated H2SO4H_2SO_4 at 170∘C170^\circ\text{C} produces ethene via intramolecular dehydration; adding water to CaC2CaC_2 yields ethyne gas via hydrolysis.
The matches correctly pair each chemical reaction with its distinctive mechanism or diagnostic test outcome: terminal alkyne acidity forming silver salts, alkene hydroxylation via Baeyer's reagent, alcohol elimination yielding ethene at elevated temperature, and carbide hydrolysis generating ethyne.

Step-by-Step Solution

1
Analyze the reaction of terminal alkynes with ammoniacal silver nitrate
but−1−ynebut-1-yne is a 1-alkyne containing a hydrogen atom bonded to an spsp-hybridized carbon (CH3CH2C≡CH \text{CH}_3\text{CH}_2\text{C}\equiv\text{CH}). This hydrogen is slightly acidic and is readily displaced by Ag+Ag^+ to form a insoluble white precipitate of silver but-1-ynide.
To distinguish terminal alkynes from internal alkynes and alkenes.
2
Analyze the reaction of alkenes with Baeyer's reagent
propenepropene (CH3CH=CH2 \text{CH}_3\text{CH}=\text{CH}_2) undergoes syn-hydroxylation with cold, dilute alkaline KMnO4KMnO_4 to form propane−1,2−diolpropane-1,2-diol. The purple trioxomanganate(VII) is reduced to brown manganese(IV) oxide (MnO2MnO_2).
To identify mild oxidation of double bonds in unsaturation testing.
3
Analyze the acid-catalyzed dehydration condition of ethanol
Heating ethanol with concentrated H2SO4H_2SO_4 at a high temperature (170∘C170^\circ\text{C}) favours intramolecular elimination of water yielding ethene (C2H4C_2H_4).
Lower temperatures (140∘C140^\circ\text{C}) yield ethoxyethane instead, so temperature controls product selectivity.
4
Analyze the laboratory preparation of ethyne
Calcium dicarbide (CaC2CaC_2) reacts directly with water to yield ethyne gas (C2H2C_2H_2) and calcium hydroxide (Ca(OH)2Ca(OH)_2).
This is the primary laboratory synthesis method for ethyne.

Key Concept

Chemical reactions, laboratory preparation methods, and diagnostic unsaturation tests for alkenes and alkynes.
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Topics

Alkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

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